Radiation is commonly used in the non-invasive inspection of contents of objects, such as luggage, bags, briefcases, cargo containers, and the like, at airports, seaports, and public buildings, for example, to identify hidden contraband. The contraband may include hidden guns, knives, explosive devices, illegal drugs, and weapons of mass destruction, such as a nuclear or a “dirty” radioactive bomb, for example. During radiation scanning, an object is irradiated by a radiation beam. Radiation transmitted through the object, which is attenuated to varying degrees by the contents, dependent on the densities of the materials through which the radiation beam passes, is detected. Higher density materials or regions within the object attenuate radiation more than less dense materials, resulting in darker or lighter regions on an X-ray image, depending on how the detected radiation is processed and displayed.
One common inspection system is a line scanner, where the object to be inspected is passed between a stationary source of radiation, such as X-ray radiation, and a stationary detector. The radiation is collimated into a fan beam or a pencil beam. The radiation transmitted through the object is detected and measured. Radiographic images of the contents of the object may be generated for inspection. The images show the shape, size and varying densities of the contents.
Fissionable, fissile, and fertile materials (“nuclear materials”) have high atomic numbers (Z) and high densities. Their presence, therefore, causes high attenuation of a radiation beam passing therethrough. Fissile materials, such as uranium-235 (Z=92), uranium-233 (Z=92), and plutonium-239 (Z=94), may undergo fission by the capture of a slow (thermal) neutron. Fissionable materials include fissile materials, and materials that may undergo fission by capture of fast neutrons, such as uranium-238. Fertile materials may be converted into fissile materials by the capture of a slow (thermal) neutron. Uranium-238, for example, may be converted into plutonium-239. Thorium-232 (Z=90), for example, may be converted into uranium-233. Fissionable, fissile, and fertile material are referred to herein as “nuclear material.” Special Nuclear Material (“SNMs”), which more readily undergo fission than other fissile materials, are defined by the U.S. Nuclear Regulatory Commission to include plutonium, uranium-233, and uranium enriched in the isotopes of uranium-233 or -235. All of these materials have densities of about 20 g/cm3.
Radioactive materials, certain of which may have lower atomic numbers and densities than nuclear materials (cobalt-60, for example, has an atomic number of 27 and a density of about 9 g/cm3), are typically shielded during shipping by high atomic number materials, such as lead (Z=82) or tungsten (Z=74). Lead has a density of about 11 g/cm3 and tungsten has a density of about 19 g/cm3. These shielding materials also cause high attenuation. Iron, which is a main material in a majority of industrial goods shipped in cargo conveyances, in contrast, which has an atomic number of 26 and a density of about 8 g/cm3, causes less attenuation. Agricultural goods, which may also be shipped in cargo conveyances, have even lower atomic numbers and densities. It is noted, however, that large amounts of even low density materials, such as agricultural goods, along a line of sight of a radiation beam, can also cause high attenuation of radiation.
While the smuggling of guns, explosives and other contraband onto planes in carry-on bags and in luggage has been a well known, ongoing concern, a less publicized but also serious threat is the smuggling of contraband across borders and by boat in large cargo containers. Only 2%-10% of the 17 million cargo containers brought to the United States by boat are inspected. “Checkpoint Terror”, U.S. News and World Report, Feb. 11, 2002, p. 52.
Standard cargo containers are typically 20-50 feet long (6.1-15.2 meters), 8 feet high (2.4 meters), and 6-9 feet wide (1.8-2.7 meters). Air cargo containers, which are used to contain a plurality of pieces of luggage or other cargo to be stored in the body of an airplane, may range in size (length, height, width) from about 35×21×21 inches (0.89×0.53×0.53 meters) up to about 240×118×96 inches (6.1×3.0×2.4 meters). Large collections of objects, such as many pieces of luggage, may also be supported on a pallet. Pallets, which may have supporting side walls, may be of comparable sizes as cargo containers and use of the term “cargo conveyance” encompasses cargo containers and pallets.
Atomic bombs and “dirty bombs,” which use a conventional explosion to disperse radioactive material over a wide territory, are examples of nuclear devices that may be smuggled in cargo conveyances and smaller objects. Radioactive, fissionable, fissile, and fertile materials that may be used to manufacture atomic devices, may also be similarly smuggled in such objects.
A variety of techniques are being used to locate nuclear devices, nuclear materials, and radioactive materials (that may not be nuclear materials), in cargo conveyances. Manual inspection of the contents of an object is too slow for regular use. Identification of radioactive materials and nuclear devices by passive inspection systems, such as a radiation detector, while faster, is difficult, because the dense materials absorb most of the photons they emit. Shielding material, such as lead or tungsten, may also be used to block the escape of radiation, preventing its detection. In addition, certain fissile materials, such as uranium-233, uranium-235, and plutonium-239 while radioactive, have exceedingly long half-lives (on the order of 104-108 years). The count rate from spontaneous decays for such material is so low, that passive detection is not reliable. Also, a relatively small amount of radioactive material may be located within a large cargo conveyance. It is also difficult to distinguish nuclear devices and nuclear materials from other dense items that may be contained within the object by standard X-ray scanning at one or multiple radiation energies.
In one example of an X-ray scanning system, U.S. Pat. No. 5,524,133 discloses scanning systems for large objects, such as freight in a container or on a vehicle. In one embodiment, two stationary sources of X-ray radiation are provided, each emitting a beam that is collimated into a fan beam. The sources face adjacent sides of the freight and the fan beams are perpendicular to each other. A stationary detector array is located opposite each source, on opposite sides of the freight, to receive radiation transmitted through the freight. In addition, X-ray radiation of two different energies are emitted by each source. One energy is significantly higher than the other. For example, energies of 1 MeV and 5 or 6 MeV may be used. A ratio of the mean number of X-rays detected at each energy endpoint by the detector array as a whole for each slice or by the individual detectors of the array is determined and compared to a look up table to identify a mean atomic number corresponding to the ratio. The material content of the freight is thereby determined.
One complication with the use of such X-ray scanning devices is that measurements of radiation after interaction with the object under inspection are statistical. The accuracy of a measurement of X-ray radiation transmitted through an object is limited by the number of photons used to make the measurement, as well as intrinsic system noise, for example. Repeated measurements of the same quantity typically yield a cluster of measurement values around a mean value. A plot of the cluster of measurements typically forms a “normal distribution” curve. The dispersion of the individual measurements (the width of the normal distribution curve) is characterized by a standard deviation. Insufficient photons may be detected to enable measurement distributions with small standard deviations. The distributions for materials of interest, such as uranium, may therefore overlap the distributions of other, non-threatening materials. It may not, therefore, be clear whether a particular measurement is indicative of a material of interest or not, resulting in either a high false positive rate or a low sensitivity.
The standard deviation decreases and the accuracy of measurement increases as more photons are detected. While the number of photons detected may be increased by increasing the scanning time, it is generally not acceptable to slow the throughput rate of a typical X-ray scanning system at ports, borders, or airports, for example.
The accuracy of a scanning system seeking to identify a material, such as uranium, for example, may be characterized by its “sensitivity” and its “specificity”. Sensitivity is the probability that the presence of uranium in a cargo conveyance will be identified. A system with high sensitivity will identify more true positives (correct identification of the presence of uranium) and fewer false negatives (missed detection of uranium) than a system with low sensitivity. However, increased sensitivity may result in an increase in the number of false positives, which may not be acceptable. Specificity, which is a statistical measure of accuracy, is the probability that the scanning system will properly identify the absence of uranium in a cargo conveyance, for example. A system with high specificity will identify fewer false positives (identification of uranium in a cargo conveyance when it is not present), than a system with low specificity.
In U.S. Pat. No. 6,347,132 B1 (“Annis”), a high energy X-ray inspection system for detecting nuclear weapons materials is described wherein an object is scanned by a high energy X-ray fan beam or pencil beam. A detected signal is processed to identify an area of high X-ray attenuation within the object. Such high attenuation is considered to be indicative of the presence of nuclear materials.